Time and Frequency domain on vibration sensor

[m3rv89]

Hi all, I am working on a vibration analysis project and I am currectly facing some problems.

A trixial accelerometer is attached to the NI-PXI 4472 and LabVIEW is used to sample the data.

My goal is to obtain the raw data from the trixial accelerometer and display it in Acceleration, Velocity and Displacement with Time and Frequency domain. 

A vibration exciter is set at 159.2Hz with an RMS amplitude of 5m/s^2 (Acceleration), 5mm/s (Velocity), 5micro m (Displacement) which the trixial accelerometer is attached and connected to the NI-PXI 4472. Therefore, in my labview program (picture attached), I have program it in such way. The result is as shown (pictures attached).

As far as I know, the value of the Velocity and Displacement graph in Time Domain should be close to the value of 5. (Please correct me if I am wrong). Secondly, there should not be any peaks as highlighted in the FFT Velocity and FFT Velocotiy graph as shown. But the graph displays random peaks at initial frequecy (0-20Hz) whenever the program is running. Thirdly, there should be a peak at 159.2Hz on the FFT Displacment graph but the graph shows no peak. 

Please help me out as I have been scratching my head on how and where did it went wrong.

[johnsold]

It is difficult to trouble shoot from images but I will comment on some of the things I can see.

1. Set the graph mapping (or the power spectrum VI) for logarithmic or decibel scaling on the y-axis. Ther probably is a displacement peak at 159.2 Hz but is it <1% of the DC component.

2. Notice that both of the integrated signals have large DC components. If you are not interested in those components, remove them before the spectral analysis and the next integration.

3. Put a Stop button on the front panel of your VI. A while loop with False wired to the termination terminal will run forever. Stopping a VI with the Abort button is like using a tree to stop a car. It works but may have unintended consequences. (Quoting another Forum user) In particular it leaves the DAQ devices running and may cause problems when startng the next run.

Lynn

[mikeporter]

With all the Express VIs it's hard to tell what is going on in your code. This is one of the big problems with Express VIs. The other big problem is the dynamic data type because it's difficult to tell exactly what the real data type is.

But what I can see I don't understand. You are taking your acquired data (which is already acceleration, right?) and integrating it three time in parallel. This doesn't make sense. 

Integrating acceleration over time gives you velocity. Integrating velocity over time gives you displacement.

Mike...

[m3rv89]

Hi, I am well aware of the intergration part and the data obtained is in acceleration form. In the first vibration level subVI, I have set intergration type to none and selected RMS averaging mode.

[mikeporter]

...which is exactly the problem with Express VIs. You are having a problem with your code so you post a picture of it, but all that's there are 7 blue boxes that we have no idea what they are doing. Three are labeled integration, but already we know that that isn't true for one of them. What about the others? Are they really doing what they are labeled? I can't tell by looking at it, but more importantly, ***you*** can't tell either. The best you can say is how you think you remember configuring them.

Mike...

[m3rv89]

I have attached the vi. Please have a look.

[m3rv89]

Updated.

The data acquired from DAQ assistant is wired to the acceleration vibration level vi to computer the RMS value. The intergrated signal as shown in the picture is wired to the input of acceleration power spectrum vi to compute the fft of the signal. Please note that although the vibration level vi shows intergrated signal label, the signal has not been intergrated as it can be set as “none”. It simply exports the “none intergrade signal”. This configuration is only applied to the Acceleration Vibration level vi because the input signal is already in acceleration form. Therefore, the signal is exported into the Acceleration Power Spectrum vi to compute the FFT of the signal.

Similarly, the data acquired from DAQ assistant is also wired to Velocity and Displacement vibration vi respectively. The configuration of the intergration type is set to be single and double respectively and wired to the power spectrum vis respectively to compute the FFT.